1. Field of the Invention
The present invention relates to a system and a method for detecting misalignment between masks in manufacturing a semiconductor device. In more specific, the present invention relates to a system and a method for detecting a position of a misalignment measurement mark which is previously formed on a semiconductor substrate.
2. Description of the Prior Art
In manufacturing a semiconductor device with lamination layers stacked with thin films having different patterns respectively, it is greatly important to accurately align a mask (reticle) with a semiconductor substrate for forming a pattern thereon.
For such mask alignment, a commonly adapted method has steps of previously forming a misalignment measurement mark (which will hereinafter be occasionally referred as a measurement mark) in a region other than that for forming a device on a semiconductor substrate, detecting the position of the measurement mark and adjusting a mask alignment position on the basis of the detected position.
Conventional methods for detecting a position of an alignment measurement mark will hereinafter be described taking a slice level method and a correlation method for instances. In the following respective figures, the same reference numbers are given to the same portions, and the descriptions thereof are appropriately omitted.
FIG. 1 is a schematic diagram showing a conventional measurement mark position detecting system. The misalignment measurement mark position detecting system 110 shown in this figure comprises; a light source 13, a half mirror 15, a stage 70, a CCD (Charge Coupled Device) sensor 33, an A-D (Analogue to Digital) converter 35 and a control computer 110. A Si (silicon) substrate 120 is supported on the stage 70. The substrate 120 is previously provided with a measurement mark 20 which is an object to be measured. In this figure a cross section diagram of the measurement mark 20 is shown taken along a line in the X direction and portions of the substrate 120 other than the measurement mark 20 are omitted.
FIG. 2 is an enlarged view of the measurement mark 20 shown in FIG. 1. As shown in FIG. 2, the measurement mark 20 includes a SiO2 layer 23 formed on the Si substrate 120, and a SiN layer 27 which on the SiO2 layer 23 so as to protrude therefrom. The SiO2 layer 23 and the SiN layer 27 are formed in thickness of T1 and T2 respectively, and each values thereof are 1 xcexcm in this example. Two concavities C1 and C2 are formed on a surface of the SiO2 layer 23. These concavities have depth D1 and D2 of 0.12 xcexcm respectively and thus constitute steps. The SiN layer 27 is arranged such that the center thereof is positioned right in the middle of concavities C1 and C2 in the cross section view of the FIG. 2. That is, from the point of view of the SiN layer 27 the SiN layer 27 is arranged such that the center thereof is positioned right in the middle of outside edges E1, E4 of the concavities. The measurement mark 20 thus forms a symmetry shape with respect to the centerline 11 of SiN layer 27.
The position of the measurement mark can be detected by detecting the center point P1 on the top surface of the SiN layer 27. However, a typical method of detecting the point P1 includes a step of recognizing that the center point P1 of the SiN layer 27 coincides with the middle point of the outside edges E1, E4 of concavities C1, C2.
(1) Slice Level Method
Referring FIGS. 3B, 4 and 6 showing waveforms and the flow-chart of FIG. 5, a slice level method for detecting a measurement mark will be described.
First, using the system 100, a beam of light L1 having a predetermined wavelength xcex or white light is emitted from the light source 13 to irradiate the measurement mark 20 via the half-mirror 15 (step S101). A reflected beam of light L2 is generated from the measurement mark 20. The reflected beam L2 passes through the half-mirror 15 and is detected by a CCD sensor 33 (step S102). The reflected beam L2 includes a ray from the interface between Si substrate 120 and SiO2 layer 23, a ray from the surface of the SiO2 layer 23, a ray from the interface between SiO2 layer 23 and SiN layer 27 and a ray from the surface of the SiN layer 27. Since above mentioned rays interfere each other, the reflected beam L2 enters the CCD sensor 33 as the beam having various light strength dependent on each difference between the optical path lengths from these interfaces or surfaces to a pixel portion of the CCD sensor 33.
In the CCD sensor 33 pixels are arranged in a row in the x direction. Electric charges are generated from each pixel in response to the rays of the reflected beam entering the pixel. Signals from these charges are conveyed to the control computer 110 through the A/D converter 35.
The control computer 110 processes the signals supplied from the CCD sensor 33 to recognize a waveform in a diagram with a horizontal axis and a vertical axis. The horizontal axis denotes X coordinates of the measurement mark in the X direction and the vertical axis denotes strengths of the reflected beam from the measurement mark (step S103). A position coordinate of the measurement mark with respect to the substrate 120 (which will hereinafter referred to as a wafer position coordinate) is detected in a conventional way.
FIG. 3B shows a waveform diagram obtained by the control computer 110 together with the shape of the measurement mark in a cross-sectional view. As shown in FIG. 3B, each position coordinate on the horizontal axis corresponds to a positional coordinate of the measurement mark respectively. For example, edges E1 through E6 of the convexo-concave shape correspond to X1 through X6 of the waveform figure respectively.
As shown in FIG. 3B, assuming that the light strength of the reflected beam from the concavity C1 corresponding the position coordinates from X1 to X2 is rd1, and that the light strength of the reflected beam from the concavity C2 corresponding to the position coordinates form X5 to X6 is rd2, and that the light strength of the reflected beam from the other surface of the SiO2 layer 23 is r0, the following correlation exists between these strengths.
ro greater than rd1, rd2xe2x80x83xe2x80x83(1) 
rd1=rd2xe2x80x83xe2x80x83(2) 
Thus, the waveform of the reflected beam obtained from the measurement mark having a line symmetry shape in a cross section view has a concavity portion in shape in and near the region of the position coordinates from X1 to X2 and a concavity portion in and near the region of the position coordinates from X5 to X6. The entire waveform has a line symmetry shape along a line 11xe2x80x2 which passes the middle point X34 of X3 and X4 and is perpendicular to the X-axis.
Referring now to FIG. 4 and FIG. 5, a method for processing a waveform in such a symmetry shape and for detecting the position of the measurement mark 20 by means of a slice level method will be described below.
First, the position XM1 where the light strength drops most sharply in and near a region having position coordinates from X1 to X2 in the waveform figure is detected (step S104).
Similarly, the position XM6 where the light strength rises most sharply in and near a region having position coordinates from X5 to X6 in the waveform figure is detected (step S105).
Next, the middle position XM16 of the position XM1 and the position XM6 acquired at above-mentioned step is calculated (step S106).
Then, at steps similar to the above steps S104 through S0106, the position XM3 where the light strength drops most sharply in a portion having position coordinate of and near the X3, and the position XM4 where the light strength rises most sharply in a portion having position coordinate of and near X4 are detected respectively (steps S107 and S108). Then a middle position XM34 of the X3 and the X4 is calculated (step S109).
At last, the difference between XM34 and XM16 is calculated and the calculated value is outputted as misalignment (step S110).
In the example shown in the waveform diagram of FIG. 4, since the waveform of the light strength of the reflected beam has a symmetric shape, XM1 and the position coordinate X1, and XM6 and the position coordinate X6 coincide with each other respectively. Therefore, it is recognized that XM34xe2x88x92XM16=0 and that MX16 accurately coincides with the middle point of X1 and X6. As a result, the position of the measurement mark can accurately be detected, so that a mask can precisely be aligned with a substrate or a pattern previously formed thereon in a subsequent lithography process.
(2) Correlation Method
Next, a correlation method for detecting a measurement mark will be described referring to FIGS. 1, 3B and 6 showing waveforms and a flow-chart of FIG. 6.
Steps S111 through S113 of obtaining a waveform figure shown in FIG. 3B by irradiating the measurement mark with the light L1 and by detecting the reflected beam L2 with the CCD sensor 33 are substantially the same as steps S101 through S103, and each of step numbers of steps S111 through S113 is that added by 5 to each corresponding step shown in FIG. 5.
As shown in FIG. 6, a portion 11 of the waveform having the position coordinates of and near X1 is extracted. Then, the waveform portion 11 is reversed with respect to a line intersecting X1 and perpendicular to the X-axis by means of a mirror-reversing process to prepare a symmetric graphic. Data on the prepared graphic are then stored in a memory (not shown) as a reference waveform 11inv (step S114).
Next, a portion of the waveform having the position coordinates X5 to X6 and position coordinates in the vicinity hereof is compared with the reference waveform 11inv and the waveform which is most similarity to the reference waveform is detected. Then the position coordinate correspondent to the detected waveform is designated as XN6 (step S115).
Then, a middle point of XN1 corresponding to the waveform portion 11 and the position coordinate obtained at step S115 is calculated and is designated as the middle point XN16 of the position XN1 and the position XN6 (step S116).
Then, the middle point XN34 of the position X3 and the position X4 is calculated at steps similar to the above-mentioned steps S114 through S116 (steps S117 through S119).
At last, the difference between XN34 and XN16 is calculated and the calculated value is outputted as a quantity of misalignment (step S120).
By means of the correlation method described above, when a waveform obtained from the reflected beam is symmetric, XN16 coincides exactly with the middle point XN34 of the position coordinate X3 and the position coordinate X4, so that it is possible to accurately detect the position of the measurement mark 20.
However, both the slice level method and the correlation method which are described above have a problem that a mark position cannot accurately be detected when a measurement mark has a non-symmetric shape. This problem will be described in more detail below.
FIG. 7 shows an example of a misalignment measurement mark having a non-symmetric cross sectional shape. Materials and film thickness of elements constituting the measurement mark 21 shown in FIG. 7 are the same as those of the measurement mark 20 shown in FIG. 2. And the fact that SiN layer 27 is arranged right in the middle of two concavities C3 and C4 is also the same as the aforementioned measurement mark 21.
However, depths D1xe2x80x2 and D2xe2x80x2 of the two concavities C3 and C4 arranged on SiO2 layer 24 of the measurement mark 21 are different from those of the measurement mark 20. In specific, the concavities C3 and C4 are formed in depths D1xe2x80x2=0.1 xcexcm and D2xe2x80x2=0.14 xcexcm respectively. Due to such constitution the measurement mark 21 has a non-symmetric cross sectional shape with respect to the centre line 12 of SiN layer 27. For this reason, when strength distribution of the reflected beam from the measurement mark 21 is obtained at steps S101 through S103 shown in FIGS. 5 and 11, the waveform thereof is then acquired as shown in FIG. 8B because a phase of the reflected beam from the portion of the concavity C4 reverses.
When the position of the measurement mark 21 is intended to be detected using the waveform shown in FIG. 8B by means of conventional methods, following problems occur.
As shown in FIG. 9, XM1 corresponds to the position coordinate of X1 at the step of detecting the position coordinate XM1 where the strength of the reflected beam drops most sharply in and near the position coordinates from X1 to X2 (step S104 in FIG. 5).
However, XM6 corresponds not to the position coordinate X6 but to the position coordinate X5 at a step of detecting a position coordinate XM6 where the rise of the light strength is expected to be most steep in and near the position coordinates from X5 to X6 (step S105 in FIG. 5). The middle point thereof then corresponds not to the middle position of the position coordinate X1 and X6 but to the middle position of the position coordinate X1 and X5. For this reason, XM16 never coincides with the middle point XM34 of XM3 and MX4 which are obtained at steps S107 through S109, and an error occurs by a distance of XE shown in FIG. 9. As a result, this error renders it impossible to accurately detect a misalignment with a mask in a subsequent process.
As can be seen from FIG. 10, a mirror-reversed waveform 13invxe2x80x2 (not shown) of a waveform portion having position coordinates of and near X3 is most similar to a portion of the waveform having position coordinates of and near X4. The middle position XN34 of the detected position coordinates corresponds to the middle point of X3 and X4 similarly to the example of symmetric cross sectional shape.
However, a mirror-reversed waveform 11invxe2x80x2 prepared from a portion of the waveform 11xe2x80x2 having position coordinates of and near X1 is most similar to that having position coordinates of and near X5. Then the middle point XN16 thereof corresponds not to the middle point of the position coordinate X1 and X6 but to the middle point of the position coordinate X1 and X5. For this reason, as shown in FIG. 10, an error occurs by a distance of XE between XN16 and the middle pointXN34 of XN3 and MN4 obtained at steps S117 through S119 shown in FIG. 11. As a result, this error renders it impossible to precisely detect misalignment between the measurement mark 21 and a mask.
As mentioned above, according to the conventional methods, a position of a measurement mark can accurately detected when a cross sectional shape of the mark is symmetric, however, there is a problem that a position of a mark having a non-symmetric cross sectional can not be precisely detected.
It is therefore an object of the present invention to provide a mark position detecting system which can accurately detect the position of an alignment measurement mark even if the cross sectional shape thereof is not symmetric.
It is another object of the present invention to provide a method for precisely detecting the position of an alignment measurement mark even if the cross sectional shape thereof is not symmetric.
According to a first aspect of the present invention, there is provided a mark position detecting system comprising: a light emitter for emitting light to irradiate a mark for misalignment measurement, the mark being formed on a semiconductor substrate, shape information of the mark and material information of an element constituting the mark are previously given; a light detector for detecting a reflected beam of light emitted from the mark on irradiation of the light; a waveform recognition part for preparing a measured waveform on the basis of the detected result of the light detector, the measured waveform denoting strength distribution of the reflected beam according to the shape and the material of the mark; a theoretical waveform preparing part for preparing a theoretical reflected beam waveform on the basis of the shape information and the material information of the mark, the theoretical reflected beam waveform denoting theoretical strength distribution of the reflected beam which would be obtained by irradiating a desired region of the mark with the light; and a determining part for comparing the measured waveform with the theoretical reflected beam waveform to acquire positional information on a place on a surface of the substrate, the place corresponding to the portion of the measured waveform which is most similar to the theoretical reflected beam waveform and for detecting the position of the mark on the basis of the acquired the positional information.
Because the theoretical waveform preparing part prepares the theoretical reflected beam waveform on the basis of the shape information and the material information of the mark, and the determining part compares the measured waveform with the theoretical reflected beam waveform, it is possible to accurately acquire a positional information on a desired place on the substrate for specifying the mark. Therefore, the position of the mark can be precisely detected whether a cross sectional shape of the mark is, for example symmetric or non-symmetric.
The theoretical waveform preparing part may preferably prepare the theoretical reflected beam waveform of a spot at which strength of the reflected beam changes. This enables to obtain positional information on a characteristic place of the mark.
In a preferred embodiment of the present invention, the mark includes a first thin film formed of a first material on the substrate and a second thin film formed of a second material on the first film so as to protrude from the first film, the first thin film being provided thereon with a first concavity having a first depth and a second concavity having a second depth, the first concavity and the second concavity are spaced from each other, and, the second thin film being arranged in the middle of the first and second concavities, the shape information includes step information concerning a thickness of the first thin film, a thickness of the second thin film, the first depth and the second depth, the theoretical waveform preparing part prepares a first through a fourth theoretical reflected beam waveforms, the first theoretical reflected beam waveform corresponding to a first place which equivalent to an outside edge of the first concavity in view of the second thin film, the second theoretical reflected beam waveform corresponding to a second place which equivalent to an outside edge of the second concavity in view of the second thin film, the third theoretical reflected beam waveform corresponding to a third place equivalent to a first sidewall of the second thin film and the fourth theoretical reflected beam waveform corresponding to a fourth place equivalent to a second sidewall of the second thin film, the second sidewall being faced to the first sidewall, and the determining part compares the measured waveform with the first through fourth theoretical reflected beam waveforms respectively, calculates a first middle point position which is the middle point of a first edge position corresponding to the first place and a second edge position corresponding to the second place, calculates a second middle point which is the middle point of the first sidewall position corresponding to the third place and the second sidewall position corresponding to the fourth place, and determines whether any alignment occurs between the first middle point and the second middle point.
When a mark in the above mentioned shape is used and the determining part determines whether any alignment occurs between the first middle point and the second middle point, it is possible to confirm whether there is any detected error or not, so that the position of the mark can be detected with a high degree of accuracy.
According to a second aspect of the present invention, there is provided a mark position detecting system comprising: a light emitter for emitting light to irradiate a mark for misalignment measurement, the mark being formed on a semiconductor substrate, material information of an element constituting a surface portion of the mark being previously given; a spectroscope for diffracting a reflected beam of light into a ray having an arbitrary wavelength, the reflected beam being emitted from the mark on irradiation of the light; a first light detector for detecting the diffracted ray diffracted by the spectroscope; a shape information acquiring part for receiving the detected result of the first light detector and the material information, recognizing a measured diffracted ray waveform denoting strength distribution of the diffracted ray according to the shape and the material of the mark and for acquiring shape information of the mark by analyzing the measured diffracted ray waveform; a second light detector for detecting the reflected beam, the reflected beam being light emitted from the light emitter and reflected on the mark; a waveform recognition part for preparing a measured waveform on the basis of the detected result of the second light detector, the measured waveform denoting strength distribution of the reflected beam according to the shape and the material of the mark; a theoretical waveform preparing part for preparing a theoretical diffracted ray waveform which is a theoretical waveform of the diffracted ray on the basis of the material information, for supplying the theoretical diffracted ray waveform to the shape information acquiring part and for preparing a theoretical reflected beam waveform on the basis of the shape information given from the shape information acquiring part and the material information, the theoretical reflected beam waveform denoting theoretical strength distribution of the reflected beam which would be obtained by irradiating a desired region of the mark with the light; and a determining part for comparing the measured waveform with the theoretical reflected beam waveform to acquire positional information on a place on a surface of the substrate, the place corresponding to the portion of the measured waveform which is most similar to the theoretical reflected beam waveform, and for detecting the position of the mark on the basis of the acquired the positional information.
According to the second aspect, the mark position detecting system further comprises the shape information acquiring part, so that the shape information of a misalignment mark can also be acquired with a single system. Therefore, it is possible to detect a position of the mark with high throughput.
It is advantageous that the mark position detecting system in the second aspect of the invention further comprises a parameter calculating part for generating a plurality of parameters capable of being candidates to the shape information and for supplying the parameters to the shape information acquiring part, wherein the theoretical waveform preparing part prepares the theoretical diffracted ray waveform on the basis of the material information every the parameter, and the shape information acquiring part compares the measured diffracted ray waveform with the theoretical diffracted ray waveform of every the parameter, selects the theoretical diffracted ray waveform which is most similar to the measured diffracted ray waveform of the theoretical diffracted ray waveforms and determines the parameter of the selected theoretical diffracted ray waveform as the shape information.
According to a third aspect of the present invention, there is provided a method of detecting a mark position, the mark being formed on a semiconductor substrate for misalignment measurement, the method comprising steps: acquiring material information on an element constituting the mark; acquiring shape information on the mark; irradiating the mark with light; detecting a reflected beam of light emitted from the mark on irradiation of the light; acquiring a measured waveform denoting strength distribution of the reflected beam according to the shape and the material of the mark on the basis of the detected result of the reflected beam; preparing a theoretical reflected beam waveform on the basis of the shape information and the material information on the mark, the theoretical reflected beam waveform denoting theoretical strength distribution of the reflected beam which would be obtained by irradiating a desired region of the mark with the light; comparing the measured waveform with the theoretical reflected beam waveform to acquire positional information on a place on a surface of the substrate, the place corresponding to the portion of the measured waveform which is most similar to the theoretical reflected beam waveform; and detecting the position of the mark on the basis of the acquired the positional information.
According to the third aspect of the invention, a theoretical reflected beam waveform is prepared the basis of the shape information and the material information on the mark, so that it is possible to accurately acquire a positional information on a desired place on the substrate for specifying the mark. Therefore, the position of the mark can be precisely detected whether a cross sectional shape of the mark is, for example symmetric or non-symmetric.
In the mark position detecting method the step of acquiring shape information on the mark may preferably include steps;
diffracting the reflected beam into a ray having an arbitrary wavelength and detecting the diffracted ray in accordance with a surface shape and a material of the mark, recognizing a measured diffracted ray waveform denoting strength distribution of the diffracted ray and acquiring the shape information on the mark by analyzing the measured diffracted ray waveform.
Thus, the shape information of the misalignment mark can also be acquired in a series of steps, it is possible to detect a position of the mark with high throughput.
In a preferred embodiment of the method of detecting a mark position, the step of acquiring the shape information includes steps of; generating a plurality of parameters capable of being candidates to the shape information, preparing a theoretical diffracted ray waveform on the basis of the material information every the parameter, the theoretical diffracted ray waveform being a theoretical waveform of the diffracted ray, comparing the measured diffracted ray waveform with the theoretical diffracted ray waveform of every the parameter, selecting the theoretical diffracted ray waveform which is most similar to the measured diffracted ray waveform of the theoretical diffracted ray waveforms, and determining the parameter of the selected theoretical diffracted ray waveform as the shape information.
Furthermore, in a further preferred embodiment of the method of detecting a mark position, the mark includes a first thin film formed of a first material on the substrate and a second thin film formed of a second material on the first film so as to protrude from the first film, the first thin film being provided thereon with a first concavity having a first depth and a second concavity having a second depth, the first concavity and the second concavity are spaced from each other, and, the second thin film being arranged in the middle of the first and second concavities, the shape information includes step information concerning a thickness of the first thin film, a thickness of the second thin film, the first depth and the second depth, the step of preparing theoretical reflected beam waveform is a step of preparing a first through a fourth theoretical reflected beam waveforms, the first theoretical reflected beam waveform corresponding to a first place which equivalent to an outside edge of the first concavity in view of the second thin film, the second theoretical reflected beam waveform corresponding to a second place which equivalent to an outside edge of the second concavity in view of the second thin film, the third theoretical reflected beam waveform corresponding to a third place equivalent to a first sidewall of the second thin film and the fourth theoretical reflected beam waveform corresponding to a fourth place equivalent to a second sidewall of the second thin film, the second sidewall being faced to the first sidewall, and
the step of detecting the position of the mark is a step of calculating a first middle point position which is the middle point of a first edge position corresponding to the first place and a second edge position corresponding to the second place, calculating a second middle point which is the middle point of the first sidewall position corresponding to the third place and the second sidewall position corresponding to the fourth place, and determines whether any alignment occurs between the first middle point and the second middle point.
When a mark in the above mentioned shape is used and it is determined whether any alignment occurs between the first middle point and the second middle point, it is possible to confirm whether there is any detected error or not, so that the position of the mark can be detected with a high degree of accuracy.